Conventionally, the price information on price tags in shops is always changed manually when the price of the product is changed. The new prices are printed out on paper or a corresponding material, and these tags with their new price markings are placed manually in a location reserved for the price tags on shelves in the sales premises. Thus, an employee must first find the correct location of the price tag to be updated, after which the previous price tag is removed and discarded and the new price tag is inserted in its position. A disadvantage in this arrangement is, among other things, the fact that the arrangement is very laborious and there is a high risk of mistakes. In case of a mistake, a situation may, for example, occur, in which the price information on the price tags on the shelves conflicts with the price information in the cash register system.
To avoid the above-mentioned drawbacks, electronic systems have been developed, in which electronic display units and their electronic displays are provided on the front edge or above the shelves, close to the products, in which the price information of the products can be changed in a centralized manner from the control centre of the system, or the like. This will facilitate and accelerate the updating of the price information to a significant extent. The data on the displays can be updated in a cabled or wireless manner, depending on the system. Cabled systems involve the problem that a wire connection must be provided for each display for data transmission and possibly also for power supply. For example in a normal retail outlets for daily consumer goods, the number of displays is relatively large; consequently, there must also be a large number of wires, which causes problems and limitations, for example, on the placement of the price displays.
Wireless systems do not require complex wirings but, in turn, they require transponders equipped with antennas in connection with battery-driven price displays, for communicating with the control unit of the system via suitable communication means.
When the aim is to minimize the power consumption of the display units and thereby to extend the service life of their batteries, one method in the communication between the communication means and the display units is such in which the transducer placed in the display units does not, by itself, implement active radio transmission so that the transmission power would be supplied from a separate power supply of the display unit, but the display unit only reflects the radio transmission of the communication means at a given moment of time and in a given way. The display unit can change the reflectance of its own antenna, wherein the display unit is capable of acknowledging or responding to messages of the communication means by simple reply messages. In the following, such a communication method will be described on a general level.
The communication means communicate with the display units by transmitting signals which are listened to by all the display units and from which a single display unit identifies a command relating to itself on the basis of a given display unit specific identification. After receiving a command relating to itself, the single display unit responds to the communication means by reflecting the transmission of the communication means itself back to the communication means, changed by a phase shift caused by a given time delay and at a given moment of time. Typically, after transmitting a command intended for a given price display, the communication means starts to transmit, for example, a carrier wave of a given type for a given period of time, the carrier wave being reflected back, with a given phase shift, by the price display that recognized the command. Consequently, the communication means is capable of identifying the reflected response as belonging to a given price display, because it knows to expect it within a given period of time after sending a unique command to said display. From its own transmission, the communication means can separate the reflected response that is significantly weaker in power, on the basis of the phase shift produced by the display device.
Because these reflected response signals are typically very weak and they can be disturbed and suppressed by other ambient reflections or signals, there is a need to reduce the effect of other interference sources on this communication by all possible means.
One of the typical interference sources in many premises is fluorescent lamps and the electronics associated with them. Particularly in shops and supermarkets, there are normally a large number of fluorescent lamps, so that the problem caused by them is significant in such arrangements. Other mains driven devices may also cause strong electromagnetic interference that suppresses weak reflected signals.
In the case of fluorescent lamps, for example, the interference caused by them may cover 0.5 to 20% of all the time used.